Depth perception is the ability of a human being to perceive three dimensions in the field of vision, including the distance at which a viewed object may be from the viewer. Depth perception is achieved in part by stereopsis, which is a process by which two slightly different images received by two eyes can be combined to assess depth and distance information by the brain. Stereopsis plays a greater role in depth perception when a viewed object is not moving or moving very slowly and also plays a greater role in depth perception when a viewed object is relatively close to the viewer. If a person lacks vision in one eye, the person can still see using the remaining eye but will lack a complete ability to perceive depth. Depth perception plays a key role in a variety of human activities including cutting foods, pouring liquids into a cup, playing sports involving catching and throwing, parking a car, sewing, firefighting, shaking hands, and aligning objects.